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Title:
METHOD FOR MEASURING DAMAGE OF A SUBSTRATE CAUSED BY AN ELECTRON BEAM
Document Type and Number:
WIPO Patent Application WO/2018/044164
Kind Code:
A2
Abstract:
A method for measuring damage (D) of a substrate (1) caused by an electron beam (2). The method comprises using an atomic force microscope (AFM) to provide a measurement (S2) of mechanical and/or chemical material properties (P2) of the substrate (1) at an exposure area (la) of the electron beam (2). The method further comprises calculating a damage parameter (Sd) indicative for the damage (D) based on the measurement (S2) of the material properties (P2) at the exposure area (1a).

Inventors:
SADEGHIAN MARNANI, Hamed (Anna van Buerenplein 1, 2595 DA 's-Gravenhage, NL)
Application Number:
NL2017/050572
Publication Date:
March 08, 2018
Filing Date:
August 30, 2017
Export Citation:
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Assignee:
NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETENSCHAPPELIJK ONDERZOEK TNO (Anna van Buerenplein 1, 2595 DA 'S-Gravenhage, NL)
International Classes:
G01Q30/02; G01N23/225; G01N29/04; G01Q60/32
Other References:
STEVENS, J. PHYS. CHEM. LETT., vol. 113, 2009, pages 18441 - 18443
Attorney, Agent or Firm:
JANSEN, C.M. (V.O, P.O. Box, 2508 DH Den Haag, 87930, NL)
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Claims:
CLAIMS

1. A method for measuring damage (D) of a substrate (1) caused by an electron beam (2), the method comprising

- using an atomic force microscope (AFM) to provide a

measurement (S2) of mechanical and/or chemical material properties (P2) of the substrate (1) at an exposure area (la) of the electron beam (2); and

- calculating a damage parameter (Sd) indicative for the damage (D) based on the measurement (S2) of the material properties (P2) at the exposure area (la). 2. The method according to claim 1, wherein the damage parameter

(Sd) is calculated based on material properties (P2) including adhesive and/or viscoelastic material properties.

3. The method according to any of the preceding claims, wherein the damage parameter (Sd) is calculated based on a comparison of the material properties (P2) at the exposure area (la) with reference properties (Pi) measured at an unexposed area where the electron beam (2) has not interacted with the substrate (1). 4. The method according to any of the preceding claims, wherein a damage severity is calculated by comparison of the damage parameter (Sd) with a predetermined threshold difference between the material properties (P2) of the exposure area (la) and reference properties (Pi). 5. The method according to any of the preceding claims, wherein the material properties (P2) are calculated based on a force-distance

measurement (200) of the atomic force microscope (AFM).

6. The method according to any of the preceding claims, wherein the damage parameter (Sd) is calculated based on one or more of

- an elasticity or stiffness (K) of the exposure area (la);

- an adhesive property (Fadh) of the exposure area (la);

- a deformation property (Zdef) of the exposure area (la);

- a peak force property (Fmax) of the exposure area (la);

- a viscosity or energy dissipation property (ΔΕ) of the exposure area (la). 7. The method according to any of the preceding claims, wherein the atomic force microscope (AFM) includes an ultrasound generator (20).

8. The method according to any of the preceding claims, wherein the atomic force microscope (AFM) is used to measure subsurface material properties (P2) below an exposure area (la) of the substrate (1).

9. The method according to any of the preceding claims, wherein the damage parameter (Sd) is calculated based on measurement of a contact stiffness (Kc) of the atomic force microscope (AFM) at the exposure area (la).

10. The method according to any of the preceding claims, wherein ultrasound waves (US) in the substrate (1) are coupled via an AFM tip (11) to an AFM cantilever (12) causing vibration of the AFM cantilever (12), wherein a vibrational amplitude (A) of the AFM cantilever (12) depends on a contact stiffness (Kc) of the AFM tip (11) contacting the substrate (1), wherein the damage parameter (Sd) is based on a measurement (S2) of the vibrational amplitude (A) of the AFM cantilever (12).

11. The method according to any of the preceding claims, wherein a contact resonance frequency (fcr) of the AFM cantilever (12) while the AFM tip (11) contacts the substrate (1) is a measure for the material properties (P2) of the substrate (1), wherein ultrasound waves (US) are applied the substrate (1) modulated by a modulation frequency (fm) near the contact resonance frequency (fcr) of the AFM.

12. The method according to any of the preceding claims, wherein the modulation frequency (fm) near the contact resonance frequency (fcr) causes an amplitude increased of the AFM cantilever (12) vibrations by a factor of two or more compared to an off-resonant vibration of the AFM cantilever (12).

13. The method according to any of the preceding claims, wherein the contact resonance frequency (fcr) shifts depending whether the AFM tip (11) contacts an exposure area (la) of the electron beam or an unexposed area (lb) of the substrate (1), wherein the shifting of the contact resonance frequency (fcr) causes an amplitude difference (dA) of the cantilever vibrations caused by the ultrasound waves (US) between the exposure area (la) and the unexposed area (lb), wherein the amplitude difference (dA) is used for calculating the damage parameter (Sd).

14. A method for metrology or inspection of a substrate (1), the method comprising

- performing the metrology or inspection by means of an electron beam (2) directed at an exposure area of the substrate (1);

- applying the method according to any of the preceding claims for measuring any damage (D) of the substrate (1) caused by the electron beam (2) at the exposure area (la); and

- comparing the measured damage (D) to a threshold; and - keeping or discarding the substrate (1) depending on the comparison.

Description:
Title: METHOD FOR MEASURING DAMAGE OF A SUBSTRATE CAUSED BY AN ELECTRON BEAM

TECHNICAL FIELD AND BACKGROUND

The present disclosure relates to a method wherein an electron beam interacts with a substrate, in particular to a method for measuring any substrate damage that may be caused as a result of the electron beam interaction.

For example, scanning electron microscopy (SEM) or electron beam (ebeam) inspection are used in semiconductor industry for metrology and inspection, e.g. critical dimension (CD-SEM) or defect review (DR-SEM) measurements. Current challenges for ebeam inspection and SEM

metrology may include for example:

1. Ever smaller feature sizes (e.g. below 10 nm) and increasing complexity of device design (e.g. 3D).

2. Application of new materials such as low k (stiffness), new resist, etcetera which can be very electron sensitive and cause shrinkage or other damage to the materials, especially those for which increasing the resolution higher electron energy is needed.

3. Introduction of new inspection methods by electron beams employing higher electron energy which allows the electrons to penetrate deeper for looking through some layers, but may also increase damage to sensitive layers.

These and other challenges make it important to accurately measure the level of damage to the device or layers when electron beams are being used. For example, Stevens at el. (J. Phys. Chem. Lett. 2009, 113, 18441-18443) describes Nanoscale Electron Beam Damage Studied by Atomic Force Microscopy (AFM). This paper discloses that there is a correlation between the depth of a depression (difference in height between the damaged and undamaged surface) created by the beam damage with both time of exposure and probe current. However, not all types of damage may be correlated to a changing surface topology, especially when the damage occur deeper below the surface.

It is desired to provide a more accurate method for measuring damage of a substrate caused by an electron beam, also below the surface.

SUMMAEY

In one aspect, the present disclosure provides a method for measuring damage of a substrate caused by an electron beam. The method comprises using an atomic force microscope to provide a measurement of mechanical and/or chemical material properties of the substrate at an exposure area of the electron beam. Based on the measurement of the material properties at the exposure area calculating a damage parameter may be calculated which is indicative for the damage. For example, the damage parameter is calculated based on material properties including adhesive and/or viscoelastic material properties.

It will be appreciated that the measurement of material properties by an AFM may reveal electron-induced changes to the substrate that are not otherwise detectable, e.g. do not necessarily manifest as changes to the topology (height) at the substrate surface. For example material properties such as adhesiveness, friction, viscosity and/or elasticity may be critically changed even if the height of the substrate surface is relatively unaffected. Advantageously, the inventors find that the

measurement of such material properties can be more sensitive than the measurement of only topology changes. Without being bound by theory, this may be because the changes in mechanical and/or chemical material properties caused by an electron beam only manifest as topology changes (e.g. indentations) in some cases where the damage is severe and

concentrated. Accordingly, the present methods may provide a more accurate method for measuring damage of a substrate caused by an electron beam than topology measurements.

In some embodiments, the damage parameter is calculated based on a comparison of the material properties at the exposure area with reference properties measured at an unexposed area where the electron beam has not interacted with the substrate. For example, the reference properties of an unexposed area are measured at the exposure area before applying the electron beam. Alternatively, or in addition, reference properties of an unexposed area may be measured at a reference area not overlapping the exposure area. Alternatively, or in addition, the damage parameter is calculated based on a comparison of the material properties of the exposure area with predetermined reference properties. By comparing the damage parameter of exposed areas with that of an unexposed area or with predetermined reference properties, any changes caused by the electron beam can be easily quantified. For example a damage severity is calculated by comparison of the damage parameter with a predetermined threshold difference between the material properties of the exposure area and reference properties, e.g. of an unexposed area or predetermined.

Material properties indicative for substrate damage can be measured using various AFM techniques. In one embodiment, the present methods include force-distance measurements. For example, a force- distance curve can be obtained by varying a distance between the AFM tip and the substrate surface while measuring a corresponding force at each distance. Typically, the curve may follow a different path depending on whether the tip approaches or retracts from the substrate surface. For example, a directional coefficient or derivative of the curve, e.g. at its maximum value or otherwise, may be used as a measure for elasticity or stiffness of the material at the area under the AFM tip. For example, a negative (pulling) force that occurs while retracting the tip from the surface, this be used as a measure for an adhesive property of the material at the area under investigation. For example, a distance that the tip can be

(reversibly) pressed into the substrate surface, may be used as a measure for a deformation property (deform abihty) of the material at the area under investigation. For example, a maximum force in the curve as the tip presses into the substrate surface may be uses as a measure for a peak force property of the material at the area under investigation. For example, an area between the curves for approaching and retracting the tip may be used as a measure for a viscosity or energy dissipation property of the material at the area under investigation. Also other characteristics of a force-distance curve, or combination of characteristics can be used as measure for material properties.

Damage caused by the electron beam may not be limited to the substrate surface, e.g. manifest additionally or exclusively as damage inside the substrate below the exposure area. Accordingly, in some embodiments the AFM may be configured to measure subsurface material properties. For example, the atomic force microscope may be combined with an ultrasound generator. Ultrasound waves traversing the substrate may interact with subsurface features. This interaction can be measured by an AFM coupling to the surface, e.g. via contact between the tip and surface. For example, ultrasound waves in the substrate are coupled via an AFM tip to an AFM cantilever causing vibration of the AFM cantilever.

In some embodiments, the damage parameter is calculated based on measurement of a contact stiffness or wave scattering of the atomic force microscope at the exposure area. For example, a vibrational amplitude of the AFM cantilever depends on a contact stiffness of the AFM tip contacting the substrate. In some embodiments, the contact stiffness may be indicative for the material properties at and or below the substrate surface. For example, the damage parameter may be based on a measurement of vibrational amplitude of the AFM cantilever. In some embodiments, a contact resonance frequency of the AFM cantilever while the AFM tip contacts the substrate is a measure for the material properties of the substrate at or below the surface. Other or further embodiments may include heterodyne force microscopy, atomic force acoustic microscopy or force modulation microscopy. In some embodiments, ultrasound waves through the substrate are modulated by a modulation frequency near a contact resonance frequency of the AFM. For example, the modulation frequency near the contact resonance frequency causes an amplitude increased of the AFM cantilever vibrations by a factor of two or more compared to an off-resonant vibration of the AFM cantilever. The contact resonance frequency may shift depending whether the AFM tip contacts an exposure area of the electron beam or an unexposed area of the substrate. Accordingly, a shifting of the contact resonance frequency may causes an amplitude difference of the cantilever vibrations caused by the ultrasound waves between the exposure area and the unexposed area. The amplitude difference can be used in some embodiments for calculating the damage parameter.

Further aspects of the present disclosure may provide an improved method for metrology or inspection of a substrate. In one embodiment, the method comprises performing the metrology or inspection by means of an electron beam directed at an exposure area of the substrate and applying the method as described herein for measuring any damage of the substrate caused by the electron beam at the exposure area. For example, the measured damage may be compared to a threshold and a decision is taken on keeping or discarding the substrate depending on the comparison. BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the apparatus, systems and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawing wherein:

FIGs 1A-1C schematically shows steps of an embodiment for measuring electron beam damage using an AFM;

FIG 2 schematically shows a force distance curve (top) in various phases of an AFM tip approaching and retracting from the substrate

(bottom);

FIG 3 schematically shows an embodiment of an ultrasound AFM for measuring subsurface material properties;

FIG 4 schematically shows curves indicative of vibrational amplitude depending on a contact resonance frequency.

DESCRIPTION OF EMBODIMENTS

In some instances, detailed descriptions of well-known devices and methods may be omitted so as not to obscure the description of the present systems and methods. Terminology used for describing particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and/or" includes any and all combinations of one or more of the associated hsted items. It will be understood that the terms "comprises" and/or

"comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be estabhshed directly or through intermediate structures or components unless specified otherwise.

The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and/or cross-section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the

description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.

FIGs 1A-1C schematically shows a method for measuring damage D of a substrate 1 caused by an electron beam 2 using an atomic force microscope (AFM). FIG 1A shows the AFM providing a measurement Si of mechanical and/or chemical material properties PI at an unexposed region lb of the substrate. FIG IB shows the electron beam 2 interacting with the substrate 1 causing damage D at the exposure area la. FIG 1C shows the AFM providing a measurement S2 of mechanical and/or chemical material properties P2 of the substrate 1 at the exposure area la of the electron beam 2. Based on the measurement S2 of the material properties P2 at the exposure area la, a damage parameter Sd is calculated which is indicative for the damage D.

In one embodiment, the damage parameter Sd is calculated based on a comparison of the material properties P2 at the exposure area la with reference properties PI measured at an unexposed area where the electron beam 2 has not interacted with the substrate 1. For example, reference properties PI of an unexposed area are measured at the exposure area before applying the electron beam. Alternatively, or in addition, reference properties PI of an unexposed area are measured at a reference area lb not overlapping the exposure area la. Alternatively, or in addition, the damage parameter Sd is calculated based on a comparison of the material properties P2 of the exposure area with predetermined reference properties PI. In some embodiments, a damage severity is calculated by comparison of the damage parameter Sd with a predetermined threshold difference between the material properties P2 of the exposure area la and reference properties PI. For example, the severity of the damage may determine whether the substrate 1 is discarded.

In some aspects, the present methods and systems may be used for metrology or inspection of a substrate. In one embodiment, such methods comprise performing the metrology or inspection by means of an electron beam 2 directed at an exposure area of the substrate 1 and applying the methods as described herein for measuring any damage D of the substrate 1 caused by the electron beam 2 at the exposure area la. For example, the measured damage D may be compared to a threshold and the substrate 1 is either kept or discarded for subsequent processing depending on the comparison.

In the embodiment shown, a laser source 13 provides a laser beam L that impinges on the cantilever 12 and reflects towards an optical detector 14. Using the optical detector 14, vibrations in the cantilever 12 can be sensed due to small deflections of the reflected beam L under influence of such vibrations. This provides an output signal 26 for further analysis, e.g. by a processor 15 to calculate the damage parameter Sd. In some

embodiments, the processor 15 may comprise a memory to store previous measurements Si or reference values for comparison.

Alternative or in addition to measuring beam deflection also other ways may be envisaged for measuring the cantilever deflection and/or vibration frequency/amplitude. Alternative sensing techniques for example include the application of a piezo-resistive layer, the electrical resistance of which vary with probe deflection. Probe deflection may in that case be detected by detecting voltage differences in an electric signal applied to the piezo-resistive layer. As another alternative, probe deflection may be detected using a piezo-electric element or layer, the potential of which changes dependent on cantilever motion. Alternatively, capacitive measurements may be applied in an electrostatic sensing technique. As some further alternatives, one may also apply an interferometer to measure probe deflection or perform a heat flux measurement in a thermal method by using a temperature difference between probe and substrate. The skilled person will be familiar with such techniques and is able to apply them in embodiments of the present invention.

FIG 2 schematically shows a force distance curve 200 in various phases (i)-(vi) of an AFM tip first approaching and then retracting from the substrate (shown on the bottom).

In phase (i), the AFM tip start at a distance from the surface where interaction forces with the substrate are negligible. As the AFM tip approaches the surface in phase (ii), the AFM tip is attracted to the surface e.g. by Van-der-Waals-forces. This may cause deflection of the AFM cantilever towards the substrate registered here as a negative force. As the AFM tip is brought even closer to the surface in phase (iii), repulsive interactions, e.g. ionic or Pauli-repulsion, start to play a role. The repulsive forces at some point counteract the attractive forces and may increase to maximum force in phase (iv). At this point, the movement may be reversed and the AFM tip retracted from the surface. As the tip retracts it may experience adhesive force caused by the AFM tip sticking to the surface in phase (v). Finally, as the AFM tip is pulled further away, the distance can be increased in phase (vi) until the interaction forces are again negligible.

In some embodiments a damage parameter is calculated based on material properties including adhesive and/or viscoelastic material properties. For example, material properties are calculated based on a force- distance measurement 200 of the atomic force microscope AFM, as shown, or otherwise. In one embodiment, the damage parameter is calculated based on an elasticity or stiffness K of the exposure area la. In another or further embodiment, the damage parameter is calculated based on an adhesive property Fadh of the exposure area la. In another or further embodiment, the damage parameter is calculated based on a deformation property Zdef of the exposure area la. In another or further embodiment, the damage parameter is calculated based on a peak force property Fmax of the exposure area la. In another or further embodiment, the damage parameter is calculated based on a viscosity or energy dissipation property ΔΕ of the exposure area la. In another or further embodiment,

FIG 3 schematically shows an embodiment of an AFM system comprising an ultrasound generator 20 configured to generate ultrasound waves (US) in the substrate 1.

In the shown embodiment of the AFM, a probe 10 is attached to a scan head 39. The scan head 39 enables scanning of the probe 10 relative to a surface of substrate 1. The probe 10 consists of a cantilever 12 and a probe tip 11. During scanning, the probe tip 11 is brought in contact with the surface of the substrate 1. For example the probe tip 11 may be scanned across the surface of the substrate 1 in contact mode (continuous contact between the probe tip 11 and the surface of the substrate 1) or tapping mode (periodic contact between the probe tip 11 and the surface of the substrate 1 during each cycle of a vibration applied to the cantilever 12).

In some embodiments, the AFM is configured to measure subsurface material properties PI, P2 below unexposed or exposed surface areas of the substrate 1. In one embodiment, the AFM tip 11 is brought in contact with the area under investigation la. In another or further embodiment, the damage parameter Sd is calculated based on measurement of a contact stiffness Kc of the atomic force microscope AFM at the exposure area la. Typically, ultrasound waves US in the substrate 1 may be coupled via the AFM tip 11 to the AFM cantilever 12 causing vibration of the AFM cantilever 12. For example, a vibrational amplitude "A" of the AFM cantilever 12 may depend on a contact stiffness Kc of the AFM tip 11 contacting the substrate 1. Contact stiffness Kc may be quantified e.g. as the combined stiffness of the tip contacting the substrate, e.g. clerivate of a force experienced by the tip as a function of displacement of the tip. It will be appreciated that the contact stiffness Kc may depend on material properties P2 below the substrate 1 surface. In turn, the contact stiffness may determine vibrational modes in the AFM cantilever 12.

In some embodiments a contact resonance frequency "fcr" of the AFM cantilever 12 may depend on the contact stiffness Kc. Accordingly, a contact resonance frequency "fcr" of the AFM cantilever 12 while the AFM tip 11 contacts the substrate 1 can be a measure for the material properties P2 of the substrate 1 at or below the surface. The contact resonance frequency "fcr" may be probed e.g. by including a modulation frequency "fm" in the ultrasound waves US through the substrate 1. Alternatively, or in addition, ultrasound waves may be generated at the tip (not shown), or both at the tip and the sample simultaneously (not shown). For example, the ultrasound waves US can be modulated by a modulation frequency "fm" near a contact resonance frequency "fcr" of the AFM. The closer the modulation frequency "fm" is to the contact resonance frequency "fcr", the higher the amplitude "A" of the resulting vibration in the AFM cantilever 12 at that frequency. Accordingly, in some embodiments, the damage

parameter Sd may be based on a measurement S2 of vibrational amplitude "A" of the AFM cantilever 12.

In addition to the modulation frequency "fm", the ultrasound waves US may comprise other signal components, e.g. a carrier frequency "fc". For example, the carrier frequency "fc" can be a relatively high frequency determining interaction with the substrate material while the modulation frequency "fm" is at a relatively low frequency near a contact resonance frequency of the cantilever. For example, the carrier frequency "fc" is more than 10 MHz, e.g. between 20 and 100 MHz. For example, the modulation frequency "fm" is lower than the carrier frequency "fc", e.g. by a factor of at least ten, e.g. between 10 kHz and 5 MHz. Of course also other frequencies can be envisaged depending on the particulars of the system under investigation and/or intrinsic properties of the cantilever.

Ultrasonic force microscopy may for example performed by applying an ultrasonic signal to the substrate and modulating the ultrasonic wave with a modulation frequency "fm" of approximately the cantilever resonance frequency. By sensing the output signal at the modulation frequency and analyzing the amplitude and/or phase, subsurface structures can be imaged. Without being bound by theory, this may be explained by the fact that the high frequency (fc) ultrasonic signal may be perturbed by the subsurface structures. Information on the subsurface structures is conveyed via these perturbations and becomes measureable in the deflection of the probe tip, i.e. the output sensor signal at or near the cantilever resonance frequency. In the embodiment shown, a signal generation and analysis system 30 is used to generate and extract signals. A first signal generator 31 provides a first signal at the carrier frequency "fc". A second signal generator 32 provides a second signal at the modulation frequency "fm". The frequencies may serve as input for a mixer 33 which generates mixed signals e.g. providing three frequency components: the carrier frequency fc, the carrier frequency fc lowered by the modulation frequency "fm" to obtain a frequency component fc-fm, and the carrier frequency fc increased by the modulation frequency "fm" to obtain a frequency component fc+fm. For example, offering these frequency component signals in a favorable signal component ratio (e.g. fc : (fc-fm) : (fc+fm) = 1 : 0.5 : 0.5) may yields an amplitude modulated wave having a frequency "fc" wherein the amplitude modulates at a frequency "fm".

In the embodiment shown, a single ultrasound generator 20 (transducer) is shown to generate ultrasound waves US at a particular set of frequencies. Alternatively, or in addition, multiple ultrasound generators (not shown) can be used in homodyne or heterodyne configuration. For example an additional frequency may be applied directly to the AFM probe, e.g. by a modulated laser beam L or otherwise. Furthermore, signals may be generated at alternative or additional frequencies than shown or only at a single (modulation) frequency. In some embodiments, the signals may be amplified in a power amplifier (not shown) before being provided to the generator 20. In the shown embodiment, a coupling medium 21 (e.g. water) is used to provide for acoustic coupling between the generator 20 and the substrate 1. In alternative embodiments this may be omitted. In the shown embodiment, the ultrasound generator 20 is below the substrate 1. This has an advantage that the ultrasound waves US are affected by the material properties between the ultrasound generator 20 and the point of

measurement (at the AFM tip). Alternatively, the ultrasound generator 20 may be positioned elsewhere. In the embodiment shown, the laser 13 sends a light beam "L" at a position on the AFM cantilever 12. Vibrational movement of the AFM cantilever 12 causes deflection of the reflected beam which is measure by sensor 14 which is sensitive to the position of the impinging beam, e.g. a quadrant detector. The sensor 14 results in a measurement signal S2.

In one path, high frequency components of the signal S2 are extracted by a high pass filter 34 to the analysis system 30. In particular, the passed signal comprises a frequency component with a certain

amplitude "A" at the modulation frequency "fm". The amplitude "A" may be retrieved e.g. by a demodulator 35 using the original modulation frequency "fm" as reference. For example, the demodulator 35 may comprise a lock-in amplifier. The amplitude "A" may be processed by a processor 36 to calculate the contact stiffness Kc. The contact stiffness may be used by processor 15 to calculate the damage parameter Sd. Of course the processors 36 and 15 may also be integrated. Alternatively, or in addition, the step of calculating the contact stiffness Kc may omitted and the damage parameter Sd directly calculated from the vibrational amplitude A. Alternatively, or in addition, the contact stiffness Kc may be directly equated a to the damage parameter Sd.

In another path, low frequency components of the signal S2 are extracted by a low pass filter 37 as a measure of a distance or height "Z" between the AFM tip over the substrate surface. The measured distance may be fed into a comparator 38 together with a desired distance "Z0", e.g. corresponding to a desired average force/deflection of the probe 10. The output signal of the comparator may be used to control a height of the scan head 39 to which the probe 10 is attached.

While the present embodiment shows ultrasound waves being applied via the substrate, ultrasound AFM can also be done via tip, or a combination. Accordingly, various embodiments can be envisaged such as heterodyne force microscopy, atomic force acoustic microscopy, waveguide ultrasonic force microscopy, force modulation microscopy. Also, the ultrasound can be generated in various ways such using piezo transducers, electrostatic actuation, photo thermal actuation (e.g. via the light beam "L"), etc.

FIG 4 schematically shows curves indicative of a vibrational amplitude "A" depending on a proximity between the modulation frequency "fm" and the contact resonance frequency "fcr".

As shown, the modulation frequency "fm" near the contact resonance frequency "fcr" causes an amplitude increased of the AFM cantilever 12 vibrations. For example the amplitude may increase by a factor of two or more compared to an off-resonant vibration of the AFM cantilever 12. In one embodiment, a position of the contact resonance frequency shifts depending whether the AFM tip contacts an exposure area of the electron beam ("fcr,on") or an unexposed area of the substrate

("fcr, off). As shown in the bottom curve, at certain modulation frequencies, such shifting of the contact resonance frequency "fcr" causes an amplitude difference "dA" of the (cantilever) vibrations caused by the ultrasound waves between the exposure area la and the unexposed area lb. In one

embodiment, the amplitude difference "dA" is be used for calculating the damage parameter.

For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include

embodiments having combinations of all or some of the features described. For example, while embodiments were shown for AFM contact force measurements and/or ultrasound AFM, also alternative ways may be envisaged by those skilled in the art having the benefit of the present disclosure for measuring chemical and/or mechanical material properties. For example, other AFM techniques may be employed such as quantitative nano-mechanical (QNM) or peak force mapping. Optical, mechanical, and/or electrical components may be combined or split up into one or more alternative components. It is appreciated that this disclosure offers particular advantages to measuring electron beam damage, and in general can be applied for measuring also other types of damage. The methods as described herein are preferably applied in contact-mode type AFM. It may however also be applied using other modes such as tapping mode, although in tapping mode the duration of periodic contact moments is relatively short and proper filtering of disturbances may be needed.

Finally, the above-discussion is intended to be merely illustrative of the present systems and/or methods and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. The specification and drawings are accordingly to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim; the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; any reference signs in the claims do not limit their scope; several "means" may be represented by the same or different item(s) or implemented structure or function; any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. In particular, all working- combinations of the claims are considered inherently disclosed.